An image sensor is a semiconductor device for converting an optical image into an electrical signal. The image sensor may generally be classified as either a charge coupled device (CCD) image sensor or a complementary metal oxide silicon (CMOS) image sensor (CIS).
During the fabrication of image sensors, a photodiode may be formed in a substrate using ion implantation. As the size of a photodiode is reduced for the purpose of increasing the number of pixels without increasing chip size, the area of a light receiving portion is also reduced, thereby resulting in a reduction in image quality. Also, because stack height may not reduce as much as the reduction in the area of the light receiving portion, the number of photons incident to the light receiving portion may also be reduced due to diffraction of light, known as “airy disk”.
Related image sensors may attempt to form a photodiode using amorphous silicon (Si), or form a readout circuitry in a silicon (Si) substrate using a method such as wafer-to-wafer bonding, and then form a photodiode on and/or over the readout circuitry. The photodiode may then be connected with the readout circuitry through a metal interconnection. Because both the source and the drain on both sides of the transfer transistor may be heavily doped with N-type impurities, a charge sharing phenomenon may occur. When such a charge sharing phenomenon occurs, the sensitivity of an output image may be reduced and an image error may be generated. Also, because a photo charge might not readily move between the photodiode and the readout circuitry, a dark current may be generated and/or saturation and sensitivity may be reduced.